(i) Field of the Invention
The present invention relates to an ink for a ball point pen and a type of a ball point pen for directly reserving the ink in a ink tank or tube (thereinafter refered to as "the direct ink reserving type ball point pen").
(ii) Description of the Related Art
A ball point pen is constituted of a pen point comprising a ball and a tip holder, an ink reserving tube, a penholder and the like. According to this ball point pen, the ball of the pen point is rotated to transfer or sink an ink to or into a recording medium such as a paper, and particularly by the transfer, written traces drawn lines can be made. At this time, the excessive ink which has not been transferred or sunk adheres to the outer periphery of the tip holder, so that there occurs the so-called blobbing phenomenon that the ink drops at the start of writing or during writing, with the result that the sharp lines cannot be drawn sometimes.
In order to solve this problem, for example, Japanese Patent Application Laid-open Sho No. 5-40192 has disclosed a method in which the outer periphery of the chip holder is also coated with a water-repellent or an oil-repellent material so as to inhibit the ink from adhering to the outer periphery of the chip holder.
However, the resultant coating material is peeled off by friction with a writing material such as a paper during writing, so that there is a tendency that the blobbing phenomenon gradually begins to occur.
This principle is that the critical surface tension on the outer periphery of the chip holder is decreased so as to be lower than the surface tension of the ink, thereby controlling the wetness of the ink. However, since a ink originally has the low surface tension, the surface of a high-energy material such as a metal is wetted with the ink inconveniently. Then, the non-aqueous ink is easier to wet the metal than an aqueous ink.
Even if the above-mentioned coating is given on the surface of a low-energy material, there is only a slight difference between the critical surface tension of the outer periphery of the chip holder and the surface tension of the ink, and hence an oil-repellent effect is too poor to sufficiently prevent the blobbing.
Furthermore, in the usual non-aqueous ball point pen, the wear of the tip by the rotation of the ball is relatively low, because the ink has a viscosity of thousands to millions cps. However, if the viscosity of the ink is less than thousands cps, the improvement of lubrication by viscus properties cannot be expected, so that the tip is extremely worn, with the result that the starving of written lines or writing impossibility takes place.
In the conventional non-aqueous ball point pen, the ink contains a solvent having a low vapor pressure as a main component, and therefore, even if a cap is removed and a pen point is exposed to air, the starving or writing impossibility scarcely occurs. However, since the ink has a high viscosity of thousands to about one million mPa.cndot.s, a high writing pressure is required, so that writing feeling is heavy. In consequence, the fingers, arm and shoulder of a writer are easily tired by the writing for a long period of time. On the other hand, in an aqueous ball point pen, the ink has a low viscosity, and therefore the writing feeling is light, but the main solvent is water which easily vaporizes. Therefore, if the cap is removed and the pen point is exposed to air, the starving or writing impossibility occurs inconveniently.
Japanese Patent Application Laid-open Hei No. 1-299880 has disclosed a non-aqueous ink for a ball point pen in which the surface tension of an organic solvent is regulated and which has an ink viscosity of 50 to 2000 cP (25.degree. C.) and simultaneously possesses the characteristics of the non-aqueous ink and the aqueous ink, but this kind of ink is not always satisfactory. In recent years, some aqueous inks having a middle viscosity are on the market as inks having many advantageous features from the viewpoints of quality and economy, but these inks are poorer, than the non-aqueous ink, in a cap-off performance that the writing impossibility scarcely occurs even when the cap is removed.